Analytical ChemistryProfessor Debashis Ray

Department of ChemistryIndian Institute of Technology Kharagpur

Module 10Lecture No 48

Electrochemical Methods 2 (Contd)

Hello and welcome to this class again where we are still continuing the electrochemical

methods and now we will talk the effect of the current, effect of the current on solutions and

these solutions are your electro active solutions, so effect of the current will see how this can

be seen because these solutions are there in some electrochemical cells. So we want to see the

effect of current on solutions and these solutions are nothing but your analyte or the analytes

and electrochemical cell is there and why we are looking for that particular electrochemical

cell is that the corresponding E0 value that means the corresponding cell potential.

(Refer Slide Time: 2:54)

So once we look at the corresponding cell potential we must have some idea about the

corresponding passage of current or the cell is producing some amount of current, so the net

current in an electrochemical cell what we have defined earlier also the one is your galvanic

cell and another is your electrochemical cell. So the net current in an electrochemical cell that

which can be measured potential across the two electrodes so if you have a cell or any simple

beaker and we have two electrodes inserted within the solution of this and this particular

solution what you can have.

So electrodes are there and how we measure that means the measured potential across the two

electrodes so it is at some point cannot be the simple difference between the two electrodes as

calculated from some equation we know which is your Nernst equation. So Nernst equation

was applicable to calculate out this E0 value and now we will see whether we can directly or

theoretically that means it is the theory which can tell us that ok we can use Nernst equation

to calculate out this particular current as well as the potential for any effect of current on a

solution

(Refer Slide Time: 3:03)

So now we will see this particular one in terms of bulk electrolysis. So we just basically go

for that means we will apply current to the solution containing the analyte to clearly see the

corresponding effect of that particular current and which is being introduced directly through

the electrodes. So electrodes are there, electrodes materials are important so basically the

bulk electrolysis that means the entire electrolysis of the solution is not that the some electron

transfer is taking place on the surface of the electrode or some other electroactive material.

So we will be getting two techniques basically two methods or two processes, one will be

called as Electrogravimetry and the other is the Coulometry that means Coulometry as I told

you that is the corresponding amount of current or the electric charge which is passed through

the solution but electrogravimetry is by owing we do something because the gravimetric

estimation we know earlier. Now electricity is passed and some changes is there and the

change of the mass in terms of the corresponding depletion from the solution or deposition on

the electrode can be measured for this particular type of technique.

So we will see the effect of current on cell potential that we are just looking for and when

there is no net current in the electrochemical cell, the measured potential across the two

electrodes is no longer as I told you just now is the difference between the two electrodes

potential as we calculate out from the theoretically predicted equation which is your Nernst

equation. So what we should see that once we have the Nernst equation we have IR drop and

polarisation. So it should be considered that means the IR drop and polarization, so two

things are coming into the picture and these two things we will now discuss that when Nernst

theory or Nernst equation we are not able to that means some modification.

We should get some modification of that equation so this modification in terms of the IR drop

and second is polarization because we are talking about the corresponding amount of current

so this basically we have to operate electrochemical cell and how we can operate a particular

electrochemical cell such that what potential we should apply and the potentials larger or

smaller than that of the thermodynamic potential what we can calculate using the Nernst

equation.

So IR drop and polarization both we should consider when there is a current that means

current is there and current is passing through the electrodes and potentials larger than

thermodynamics potentials are need to operate an electrolytic cell, so when we have IR drop

and polarization. So these are the two basically reducing things that which can also go against

the corresponding potential values so we have to use a higher potential value than the

thermodynamics potential value such that we get some very small amount of current in the

cell otherwise, we will not get that particular current.

(Refer Slide Time: 6:56)

So one such example is that electrochemical cell we use for the determination of cadmium in

HCL solutions by electrogravimetry or coulometry, so we should know the corresponding cell

arrangement, so what should be the corresponding cell arrangement, that means we have to

use this for the reduction of cadmium 2 + which having some concentration and a cadmium

wire or cadmium plate can be your electrode. So what we get over there from this particular

determination that we have a cell so this is on the right hand side you have the cadmium is

getting reduced to cadmium metal.

So what we get with a certain concentration of that cadmium solution and on the left hand

you have the corresponding reference electrodes which is based on silver-silver chloride

electrodes having a particular chloride concentration of 0.2 molar. So this particular cell has a

thermodynamic potential which we can measure as – 0.73 volt. So if we consider that this

negative sign of the cell potential indicates that there is no spontaneous reaction for the

reduction of cadmium 2 + on the right and oxidation of silver on the left because the cell

convention we know that on the right you have the reduction and on the left you have the

corresponding oxidation.

So whether if it is coupled at all with that of our standard silver-silver chloride reference

electrode whether we get a corresponding process which is spontaneous for the reaction

where you get that the cadmium will be reduced to cadmium metal or the metallic cadmium

on the electrodes surface and silver will be oxidized to silver chloride in presence of certain

amount of chloride already present in the solution. So that basically gives us some ideas that

whether we will be able to use this for the corresponding determination as well as

quantitative estimation of cadmium 2 + in the solution.

(Refer Slide Time: 9:09)

So what we see that you have a corresponding cell and you get that particular cell as silver-

silver chloride electrode on the left and cadmium on the right and as I told you the

corresponding values. So if you have this and if the cell is there, we will try to measure the

potential which is there as – 0.73 volt as calculated, but if we see in the circuit that the

corresponding ammeter if it is there which can find the corresponding amount of current

which is passing through this circuit is zero.

So is 0.00 milliampere is the current so no net current is passing through the solution and

there is something that we are not getting that particular thing that means this is the

corresponding E applied and E applied potential is similar to that of your potential what is

measured from there, but due to that particular ohmic potential ohmic potential is will see that

is multiplication of the corresponding current and the resistance.

So in order to generate a current of I ampere in the cell because here we see that no more is

there so if we want to get some amount of current to be seen to be recorded from this solution

what we have to do we have apply and a potential that is IR volts more negative than the

thermodynamics cell potential, that means we have a ohmic potential and the ohmic potential

have to be added to that cell potential, so thermodynamic cell potential was – 0.73 volt

(Refer Slide Time: 11:28)

Now we will see what sort of resistance we have that I we know that I once we know the

current how much current we can pass through it and the resistance, the resistance of the cell

we should know. So if the cell of the resistance is known and then we can find out the

corresponding applied potential beyond the thermodynamic potential value so this basically

gives us these things that means if you have a R value of 15 ohm, so that is the ohmic

potential or the IR drop of the cell.

(Refer Slide Time: 11:49)

So if we have a corresponding potential value in that way, so we get basically from there, the

ohmic potential values which basically resisting the flow of charge which is nothing but a

product of I and R. So IR drop of the cell where I is the corresponding current in amperes in

amps, it can be milliamperes also and resistance R in unit of ohm. So what we get now that

we have that particular intension that how we measure the corresponding E cell as we know

in all other previous cases that E cell was E right – E left.

Now we have generate a current of I amperes in the cell so we must apply some potential

which is higher than that of the potential required for the correspond cell. So now E applied

what we should apply such that we get some current in the cell. We require some current

within the cell so E applied will be therefore that you can have this particular thing that E

applied will be E cell now this E cell – your IR drop of the cell. So we add up this IR drop in

the cell and we can calculate it out how much you have the IR drop.

So we basically this we will find out and when we discuss about the cyclic voltammetry

measurement is another electrochemical technique, so this particular one that means we can

consider as IR compensation and automatically the most modern instruments the

contemporary instruments which is available in the market for the recording of cyclic

voltammetry measurements. You can have some the corresponding adjustment in terms of

their IR drop. So IR drop is the most important factor over there to get some current over this

particular cell.

So if you have some 15 ohmic current resistance within the cell between these two electrodes

of silver and cadmium so this is the resistor we have been shown. So cell itself has a

resistance, so what we get we want to have a flow of current of I such that you can have 2

millimampere current such that you apply potential will change, so it is changing from –

0.7344 to – 0.764 volts so these increase in this corresponding E applied. The applied voltage

is due to the corresponding generated cell resistance and we consider this as the

corresponding drop in the method.

What we use for your electrogavimetric technique so this particular IR drop so we

considering is as the corresponding IR drop. How to reduce this particular IR drop, so in a

particular cell when we use and we have seen that a typical mechanism will apply goes if the

IR drop is very much your E applied will be above much more above than that of your E cell.

So what we should do. we should have some idea about the corresponding cell resistance so

how to reduce the cell resistance that resistance what we are seeing how to reduce this

particular cell resistance that is an important thing.

So if in the solution we have species or the salt which can give rise to high ionic strength or

we add something sometime we will be seeing that we will add supporting electrolytes so

these supporting electrolytes having high ionic strength can reduce the cell resistance, so IR

drop will be less. As a result we have something that we go for instead of two electrode

system because in cyclic voltammetry measurement also we will find a three electrode

system and that is very much useful for the cell it can be considered as a three electrode cell

and there the current what we have just seen that 2 milliampere current so their basically the

entire current is passes between working electrode.

So which is electrode where we just go for its nature that means is a cathode or anode,

working electrode and auxiliary electrode will put some, auxiliary electrode. So the entire

current is passing through working electrode and we are bringing something as your auxiliary

electrode and this auxiliary electrode is also known as counter electrode. That means your

reference electrode is not hampered much, the current is not passing through the reference

electrode because this is utilized again one technique is the maintaining the strength of high

ionic strength and also this particular introduction of auxiliary electrode which is a counter

electrode and which again minimizes your IR drop.

So it minimizes the IR drop, so that basically gives us some important ideas to ask that how

we can use for this and what are these ohmic potential and how these potential can be

compensated. So for this electrogavimetric method what will be see that if we go for some

measurement where you have the control potential that means potentials you have to control

for these electrogavimetric measurements. So like that of our IR drop another one the second

one what we are considering is your polarization effect, polarization effect.

(Refer Slide Time: 19:43)

So as we have seen just now that E applied is equal to E cell – IR in electrochemical cell.

Now if we see that if we want to plot the corresponding amount of current what we will be

getting that means I and that we get from that the total is E cell – E app that means the

applied potential divided by your resistance R is equal to therefore – E app taking this first

divided by R + E cell divided by R. Now what we want to do we want to plot it to see the

thing that means how the corresponding dependence that means the variation we can see. If

we try to monitor the current in milliampere and the corresponding E app, this E app will be

plotting so therefore E app is there. So E app in volt so we will starting from 0.7 volt to – 0.9

volts and current is basically 4 ampere to 8 ampere.

So what we see in this particular case that means I is plotted against I is plotted against E app

this, so we will see that means that what we will get that the corresponding magnitude of this

for your this R value how it is changing, so is basically up to certain point you have the

linearity and then it is deviating that means when the current is high, (())(21:16) 4 and it is 8

milliampere.

So at this level basically when the current is high and if we because the you have the

corresponding linear function you can have, so the deviation from this linearity can be

extrapolated like this and at say here you can have the corresponding value of these of 7

millivolts milliampere sorry 7 milliampere so at this 7 milliampere how much deviation we

get for this magnitude, this magnitude at 7 we get so this particular magnitude so here that

corresponding one where it is deviating that means this polarization behaviour or the

polarization effect is starting at this point.

So your polarization begins at this particular point and now we get the corresponding

polarization effect which is Pie that capital Pie is equal to – 0.23 volt this particular

magnitude, this particular magnitude of this potential we get, that means this basically gives

us the idea about the corresponding one because this dotted one is the corresponding

theoretical curve where you do not have any polarization that means no polarization line so

this is your no polarization line and this is giving you the corresponding degree of

polarization.

So at this point when we tried to gather a value of this 0.7 so to achieve a current of 0.7

milliampere in the electrolytic cell, we have to face this particular polarization effect in this

case, so your equation for that what we are looking at, so E app is therefore changing which

will be your E cell – IR drop the ohmic potential we have discussed earlier + your Pie value

that means your polarization effect. So this polarization effect as we have seen that this IR the

IR drop or the ohmic potential value is due to the solution of the corresponding electrode cell

system.

But this particular one the polarization is the corresponding value for your electrodes, so this

basically this is your the Pie is your electrode phenomena so it can affect either or the both

can affect either the cathode or the anode or both, either or both the electrodes, so this

polarization effect can have some interesting observation to get that means you can have this

polarization effect of two types.

One is the corresponding concentration polarization and other is the kinetic polarization so

way we all know that when we talk in terms of the electrode potential we have the

thermodynamics electrode potential but you can also have some kinetic effect the kinetic

kinetics for the electron transfer reaction. How fast and how slow is for your electron transfer

reaction whether you are talking in terms of the oxidation or the reduction reaction so you

have the concentration polarization as well as the kinetic polarization.

(Refer Slide Time: 26:18)

So when we talk in terms of the concentration this is very important aspect at this point to

understand since we have a variable concentration of the solutions, so you have some when

you have a concentration definitely we must have the concentration gradient in the solution

and that concentration gradient is very important in terms of a rate of mass transfer.

So there will be finite rate for the mass transfer because what we see that if you have the cell,

an electrode is dipped inside and that electrode you have some say if you have the

corresponding electrode responsible for the reduction of these species which are nearby but

what we will be seeing overall that if you have all these are in the solution, so those species

whether it is the species which is the responsible for reduction or the species responsible for

oxidation, which are nearby to that of your electrode are giving that particular reaction very

quickly.

But those are here but those are here they have to cross this particular concentration gradient

and from the solution basically because this is the solution to you have to go to the electrode

surface which is very important. Therefore you have some interfacial region and these

interfacial regions are very small and it can be fraction of some nanometres. So fraction of

nanometre and we are looking for that means the species that means the analyte that means

the reactant, how quickly the reactant are reaching the electrode then means mass is being

transported that means we are talking something related as a concentration gradient is there,

therefore we have to talk about the corresponding mass transfer.

So the mass transfer process is therefore is there and the reaching of reactant to the electrode

surface that means the transportation of the reactant is dependent on several factors like 1, 2

or 3 we can consider right now. One is your diffusion, second one is the migration and third

one is convection, so these are the things which are responsible for transferring the thing that

you can go for the transfer of your species for solution to the electrode surface and that is

therefore very much important then the second one.

One was concentration and the second one is your kinetic one, so kinetic one is also related to

that of your current that the magnitude of current what is getting so is basically related to the

corresponding limiting thing and where we get the rate of one or both the electrodes reactions

that means it is dependent on the electrode reactions and those electrodes reactions you have

over voltage and how to overcome that means if you have over voltage so you have to

overcome that over voltage.

To overcome that thing we have to go for that that means the activation energy to be

considered and that activation energy when we consider for this thing that that activation

energy is therefore the corresponding barrier for the different half reactions without the half

cell reactions for those electrodes. So we will see these things are together very much

important for controlling the corresponding reaction whether we go for any electrogavimetric

technique or which can be a control potential or sometime it cannot be controlled by the

potential controlling mechanism.

So the diffusion that means the when is diffusion is there that means there is a concentration

gradient and that particular concentration gradient is basically disappearing so what diffusion

is telling us that disappearance of the corresponding concentration gradient so that means

diffusion is if it is operating so we do not find any kind of cell concentration polarization. So

disappearance of the concentration gradient is there. Then for migration we must have

electrostatic attraction between the ions and the electrodes that means electrostatic are

working and between the ions and the electrodes.

So this is important that if there is some interactions because ions are charged and electrodes

are also charged so if there is some attraction so migration will be facilitated and convection

we just simply allow by doing this by using a magnetic stirrer so mechanical means of

stirring is useful so stirring is there, so that will allow the convection which will remove the

concentration gradient and also we have seen that we can use the addition of supporting

electrolyte.

So addition of supporting electrolyte reduces the cell potential and that supporting electrolyte

that means supporting electrolyte also decreases or reduces the IR drop. So these things that

means which are going against the corresponding measurement or the current transfer or the

passage of the current should we should keep in our mind that there should be some

corresponding current has diffusion control current we will call then migration and then

convection how we can use so when we see the actual practise of all these things, we will see

and we will discuss once again that how and why and what sort or supporting electrolyte we

can use such that we can decrease the corresponding IR drop within the cell, Thank you very

much